This invention relates to roofing elements and more particularly but not exclusively to roofing elements having a great span length for the construction of roofs for factory buildings and the like.
U.S. Pat. No. 1,075,845 (Mills) discloses a structural material made up of two parallel spaced wooden members united by upper and lower skins of sheet metal. This material, if used for roofing, is subjected to compression stresses which have to be taken up almost completely by the wooden members, since the sheet metal upper skin would be easily overstressed otherwise. Accordingly, the wooden members must be of ample cross dimensions, entailing a great expenditure of wood. Further, the massive wood components will form heat-conducting bridges between the inside and outside.
German Patent Specification No. 812,834 discloses a hollow all-wood construction element having both outer skins of wood-based board material and the distance members formed by composite beams having flanges of wood and webs of board material. Being an all-wood structure this element lacks fireproofing, contrary to that one afforded by a sheet metal skin. This structure will also involve a weight penalty due to the provision of two relatively thick outer skins.
Swedish Patent Specification No. 385,603 provides a construction element having an upper skin of plywood or the like and bottom members of channel-shaped sheet metal profiles bonded to the upper skin. The metal profiles will form extensive heat-conductive bridging and do only extend opposite parts of the upper plywood skin whereby fire-proofing becomes ineffectual.